1. Field of the Invention
The present invention relates to a method of measuring a focus suitable for use in an adjustment of a focussing voltage of a television receiver or a monitor using a cathode ray tube.
2. Description of the Related Art
Before a manufactured picture display apparatus using the cathode ray tube is shipped from a factory, it is necessary to adjust it to a proper condition by measuring a focus of its electron beam in order to obtain a satisfactory picture displayed on a screen of the cathode ray tube.
The assignee of this application previously proposed as one of the methods of measuring the focus a method of measuring the focus by displaying a so-called crosshatch image including a vertical line and a horizontal line crossed each other on a screen of a cathode ray tube and then reading the crosshatch image by a CCD line sensor (Japanese patent application No. 304032/1992 etc.). In this method, the focussing voltage is adjusted so that the finest width of the vertical line and the finest width of the horizontal line displayed on the cathode ray tube as the crosshatch image are detected from an output of the CCD line sensor. This adjustment allows the electron beam to be adjusted to in focus state.
However, when the focus is measured in accordance with the measuring method of detecting the widths of the vertical and horizontal lines thus displayed, the in focus state or a proper focus state can be detected, but when it is not an in focus state, it is impossible to determine whether the current adjusted state is an over-focus state (a state at a focussing voltage lower than a correct one) or an under-focus state (a state at a focussing voltage higher than a correct one). Therefore, the adjustment of the focus to the in focus state takes a long time and a high-accuracy focus adjustment is difficult to achieve.